The present invention relates to the recovery of multiphase modulated digital data, and more particularly to a method and apparatus for recovering high quality digital audio signals transmitted via a cable television system or over the airwaves.
New digital techniques for the reproduction of sound provide performance that is far superior to analog techniques which have been used in the past. An example of high fidelity sound reproduction using digital techniques can be found in the compact disc technology that has enjoyed tremendous success as an alternative to phonograph records and tapes. Digital recording and playback techniques provide reproduction of music that is extremely realistic and absent from background noise and distortions which have plagued other high fidelity sound reproduction systems in wide scale use.
Cable television ("CATV") networks have provided one medium for the transmission of digital audio programs. Cable television growth has come from the development of various programming categories and by the technologies which have made program delivery possible. Cable first brought distant television signals to areas where there was little or no off-air reception. This applies to distant signals and weak signal areas where outdoor antennas were previously mandatory. The next category to bring major growth to cable was pay service after the development of reasonable cost satellite delivery systems. After satellite delivery was accepted and less costly, super stations and cable networks formed another category of programming that has become customary, providing "extended basic" services. Franchising and local politics have created a generally unprofitable but necessary category called "local origination". Recently, addressable technology and aggressive marketing have enabled "pay-per-view" programming to proliferate.
The provision of high quality audio services over cable is rapidly becoming available. In the past, the signal quality of analog audio transmission techniques has been poor and there has been no efficient way to collect revenue or control access to such services.
Digital techniques for the communication of high fidelity audio programs via a CATV network as well as via direct broadcast satellite and over the air enable the provision of high quality audio services on a subscription basis. Commonly owned U.S. patent application Ser. No. 07/280,770 filed Dec. 6, 1988 for "Apparatus and Method for Providing Digital Audio in the FM Broadcast Band", incorporated herein by reference, discloses a method and apparatus for providing high quality digital sound signals within the FM broadcast band. Commonly owned U.S. Pat. No. 4,821,097 to C. Robbins entitled "Apparatus and Method for Providing Digital Audio on the Sound Carrier of a Standard Television Signal", also incorporated herein by reference, discloses a system wherein the FM audio portion of a standard television signal is replaced with digital audio. In the techniques disclosed in these references, the digital audio information is carried using multiphase modulation.
A well known type of multiphase modulation is quadrature phase shift keyed ("QPSK") modulation. Advantages of QPSK modulation are discussed in C. Robbins, "Digital Audio for Cable Television", 1986 NCTA Technical Paoers, Dallas, TX, Mar. 15-18, 1986, pp. 21-24.
In receiving multiphase modulated data, it is important that the data be detected at a valid point in each phase component of the received signal. In a QPSK transmission, where an in-phase component I and an out-of-phase component Q (typically, 90 degrees out of phase with the I component) are present, the data is only valid in a center portion of the stable part of each of the I and Q data inputs. In a demodulator for any multiphase modulated data, whether it be audio data or other types of transmitted data, two requirements must be met, namely, (i) the detection of valid phase comparisons must be accomplished, and (ii) the effects of periods of no data transitions must be minimized.
It would be advantageous to provide a method and apparatus that maximizes the ability to detect valid phase comparisons while minimizing the potential for data recovery errors during periods where there are no data transitions in one or more phase components of a multiphase modulated signal. It would be particularly advantageous to provide such a method and apparatus that provide a clock recovery scheme wherein a data clock is phase locked to provide a clock transition exactly in the center of the stable part of each phase component data input. A method and apparatus providing such a clock recovery scheme could be used to sample the I and Q inputs of a QPSK modulated signal to produce clean and stable I and Q data.
The present invention provides a method and apparatus enjoying the above-mentioned advantages.